CONDITION

Hemivertebrae are vertebral anomalies that can be detected sonographically by the second trimester of pregnancy. These anomalies develop during the first 6 weeks of gestation, when the future anatomical pattern of the spine is formed in mesenchyme. Once the mesenchymal pattern is established in the embryo, subsequent cartilaginous and osseous stages follow (McMaster and Ohtsuka, 1982). At approximately 6 weeks of gestation, a chondrification center appears for each mesenchymal vertebra. Each vertebral body has a dorsal and ventral primary ossification center. These centers fuse, resulting in three primary centers of ossification, which can be visualized sonographically as early as 12 weeks of gestation, but histologically as early as 8 weeks of gestation (Zelop et al., 1993). Abnormalities of the vertebral bodies result from either failure of formation or failure of segmentation (Abrams and Filly, 1985). Abnormalities in vertebral segmentation result in bar or block vertebrae, whereas abnormalities in formation result in hemivertebrae (McMaster and Ohtsuka, 1982). Nasca et al. (1975) classified hemivertebrae by their morphologic appearance and described six different types: single supernumerary hemivertebrae, single wedge-shaped hemivertebrae, multiple hemivertebrae, multiple hemivertebrae with a unilateral bar defect on the contralateral side, balanced hemivertebrae, and posterior hemivertebrae. This latter defect occurs when the anterior part of the vertebral body fails to develop. Clinically this results in a kyphosis rather than a scoliosis. The medical significance of hemivertebrae is that they act as a wedge within the vertebral column, causing a curvature away from the side of the defect (Zelop et al., 1993). The abnormal vertebral body elongates the convex side of the spine. When growth occurs on the affected side, it causes compression of the superior and inferior vertebral end plates, resulting in decreased growth on the concave side (Nasca et al., 1975).

Hemivertebrae are a major cause of congenital scoliosis and kyphoscoliosis. Prenatal detection of this abnormality has become possible only within the past few years, so relatively little is known about the clinical significance of isolated asymptomatic vertebral defects.

INCIDENCE

In a study of more than 15,000 chest radiographs, the incidence of congenital scoliosis due to vertebral anomalies was 0.5 in 1000 livebirths (Wynne-Davies, 1975). This estimate of vertebral anomalies is low, as only thoracic abnormalities were included in this study. The probable incidence ...